Process and apparatus for manufacture of hydroxide slurry

ABSTRACT

A method of manufacture of high-solids hydroxide slurries from caustic calcined carbonate powder is described, whereby the properties of the slurry are its low resistance to shear thinning to facilitate transport, a high stability for transport and storage, ease of reconstitution after long periods of storage, and, as required, a high concentration of chemically reactive species at the particle surface. The method achieves these specifications by mixing caustic calcined carbonate or hydroxide powder with water in an insulated reactor vessel, and agitating the slurry sufficiently such that the hydration reaction causes the water to spontaneously boil, such that the remaining hydration proceeds spontaneously under the fixed conditions of boiling through the water loss. The mixing process is preferably carried out by a shear pump. A viscosity modifier, such as acetic acid, is used to thin the slurry to enable the mixing system to maintain uniform mixing. The reaction is terminated when the boiling has spontaneously ceased and the temperature has spontaneously dropped to a set point though the reactor heat losses, where the processing time is sufficiently long that the slurry meets the desired specifications.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase entry under 35 U.S.C. § 371 ofInternational Patent Application PCT/AU2014/000979, filed Oct. 15, 2014,designating the United States of America and published in English asInternational Patent Publication WO 2015/058236 A1 on Apr. 30, 2015,which claims the benefit under Article 8 of the Patent CooperationTreaty to Australian Patent Application Serial No. 2013904096, filedOct. 24, 2013.

TECHNICAL FIELD

This disclosure broadly relates to a process and apparatus formanufacture of high-solids hydroxide slurries from caustic calcinedcarbonate powders that may be produced from calcination of magnesite,dolomite and limestone and mixtures thereof, whereby the slurries havelow resistance to shear thinning to facilitate reconstitution aftermonths of storage with mild agitation.

BACKGROUND

Hydroxide slurries are aqueous suspensions of solid hydrated oxides ofprimarily magnesium, as Mg(OH)₂, calcium Ca(OH)₂ and mixtures thereof,in water. They are widely used in many industrial processes, an exampleof which is the treatment of water to raise the pH and eliminate odors,particularly for sewerage treatment, and to precipitate heavy metals.These slurries are increasingly replacing sodium hydroxide because oftheir inherent properties.

For slurries with a high pH of about 12.0, a hydrated lime slurryCa(OH)₂ or dolime slurry Ca(OH)₂.Mg(OH)₂ is used, whereas for slurrieswith a pH of about 10.4 when diluted in water, a magnesium hydroxideslurry Mg(OH)₂ or semidolime Mg(OH)₂.CaCO₃ slurry is used. For seweragetreatment, magnesium hydroxide slurry or semidolime slurry is preferredbecause any excess magnesium hydroxide entering digesters does not killthe bacteria, whereas overdosing of sodium hydroxide or hydrated limeslurry can destroy the bacteria and close down the digestion process. Inheavy metal removal, the pH of the magnesium hydroxide or semidolimeslurry is in the desirable range where amphoteric hydroxides of toxicmetals precipitate, whereas at the pH of sodium hydroxide or hydratedlime, the initial precipitates re-dissolve.

Hydrated lime slurries are produced using lime, CaO, from a kiln, andthe hydrators for that process are known in the art. Generally, the limeis sufficiently reactive that the slurry is formed quickly, in typicallyless than sixty minutes from granules of the order of 1 mm. The processmay include grinding the lime, and generally the process requires theaddition of dispersants to provide the required stability and to assistthe slurry production process. Many of these dispersants are destroyedat high temperature, so that a lime hydrator is generally cooled.

Magnesium hydroxide slurries are produced from either precipitatedmagnesium hydroxide or from hydrating magnesium oxide, which has beengenerally produced from the calcination of the mineral magnesite.

Preferred properties of hydroxide slurries may include:

-   -   1. The slurry should contain particles that can react quickly        when the slurry is, for example, mixed with waste water. Thus, a        slurry may be composed of small particles with a median size of        5 μm and a low surface area about 4 m²/gm (as measured by gas        absorption methods of the dried powder), or a median size of 20        μm and a high surface area about 2.0 m²/gm. A wide, or multiple        peaked, particle size distribution, with a sharp cut-off, is        preferable to promote the slurry stability. Such a particle size        distribution d₉₀ may be less than 100 microns.    -   2. The chemistry of the surfaces of the particles in the slurry        are strongly dependent on the surface area of the oxide        particles that are hydrated to form the slurry, and the range of        applications of the slurry depend on these surface chemical        properties. Thus, a slurry produced from oxide particles that        have a surface area on the order of 150 m²/gm or higher have        markedly different exterior chemical surface properties than a        slurry produced from conventional oxide with a surface area of        20 m²/gm. Without being bound by theory, oxide particles having        a high surface area have larger amounts of energetic chemical        defects, such as superoxides and peroxides, at the crystal grain        boundaries, and these species largely survive hydration to        confer on the slurry particle surfaces a different degree of        surface chemistry that correlates with the surface area of the        initial oxide used to form the slurry.    -   3. The percentage of solids by weight is at least 50%, and        preferably 55-65%. The higher the solids content, then the less        water is required to be shipped. However, the higher limit is        generally the result of the requirements that the slurry has a        low resistance to thinning and, thus, a low apparent viscosity        at low shear rates. If the solids fraction is too high, a gel        tends to form that has a high resistance to thinning and a high        apparent viscosity, and is not readily usable in the        applications described above.    -   4. The apparent viscosity at ambient temperature is 50-900        centipoise (cps), preferably 50-300 cps at a shear rate of 200        rpm or less. With such a low resistance to thinning, the slurry        is regarded as thin, preferably when the apparent viscosity is        less than 100 cP. The achievement of such thin, high-solids        slurries generally requires a viscosity modifier to facilitate        the breakdown of gels with minimum agitation. The desirability        of thin slurries is that they are easily handled, and are more        amenable to the application of subsequent processing steps that        deliver desirable properties such as sprayed coatings that have        a high strength because the low viscosity delivers an ease of        application and surface coverage with a low water content that        would otherwise cause cracking and/or low strength when dried        due to the excessively high permeability from water evaporation        during drying. In many applications, this desirable property of        thin slurries is augmented by the surface chemistry that arises        from the use of high surface area oxide particles used to        produce the slurry.    -   5. The stability of the slurry is such that it can be used up to        many months after manufacture: The characterization of slurry        stability is often somewhat arbitrary and may involve meeting a        number of criteria. For example:        -   (i) One criterion may be that of pourability/flowability, so            that more than 80% by weight, preferably greater than 90% by            weight, of a bulk sample in, say, a 1-m³ container pours off            after 7 days of undisturbed gravity settling;        -   (ii) Another criterion may be that after 7 days of            undisturbed (unagitated) gravity settling, water separation,            called syneresis, in a vessel of 1 m³ container of 1 meter            depth is less than 30 millimeters; or, in 30 days, the            syneresis is less than 50 millimeters.        -   (iii) Another criterion may be that the slurry has less than            1% sediment (“heel”) after 30 days.        -   (iv) Another criterion may be that the syneresis is less            than 5% after 30 days, or preferably 3%.

It is recognized that these thin, high-solids slurries of variablechemical reactivity do not have a long intrinsic lifetime, and somedegree of sedimentation occurs. For many applications, there-suspendability of the slurry is much more critical than long-termslurry homogeneity, because agitation can be provided at the point ofstorage, and if required, such agitation during storage may beintermittent.

The choices of viscosity modifiers and stabilizers to produce stable,thin, high-solids slurries are generally associated with the surfacecharges on the particles and the ionic strength of the water. Theviscosity modifiers and stabilizers are generally not specific to themethod of manufacture of the slurry.

The prior art for the production of hydrated lime slurries from kilnlime is well understood. Of relevance to this disclosure, U.S. Pat. No.3,573,002 discloses a two-stage process, and uses steam pressure toovercome differences in the hydration rates of lime and magnesia thatotherwise cause significant issues in making slurries of mixed alkalineearth hydroxides. The use of pressure vessels adds to the complexity andcost of a slurry plant. The kiln lime is generally a burned lime inwhich the granules are sintered to give a moderate surface area. This isacceptable because the hydration process of lime causes the granules tobreak up from the stresses induced as the particles expand during thereaction to accommodate the water. There is a need for a process thatcan produce hydroxide slurries from un-sintered materials without theneed for high-pressure processing. Conventional lime hydrator plants,operating at ambient pressure, require inputs of lime with low magnesiumoxide content.

The prior art for the production of magnesium hydroxide slurries thatmeet the industrial requirements listed above is characterized by theinitial solids materials used to make the slurry. It is noted that theseprocesses do not generally use the approach of using high-pressuresteam, as disclosed for hydrated lime with magnesia, to accelerate themagnesia hydration reactions. The magnesia particles generally do notexhibit significant fracturing from the hydration processes. Theseclasses of materials for magnesium oxide slurries are:

a) Precipitated Magnesium Hydroxide (PMH).

PMH is generally produced by the precipitation of magnesium hydroxidefrom brines by the addition of hydrated lime. The prior art, describedbelow, focuses on the use of (i) viscosity modifiers that thin theslurry and (ii) stabilizers that facilitate the stability of the slurry,formed by agitating (deflocculating) the washed precipitate in water.The specific viscosity modifiers are selected to deal with the presenceof significant amounts of residual chloride ions in the washedprecipitate. PMH does not require grinding or hydration to make theslurry because the particle size of the PMH is similar to that of thedesirable slurry, namely about 25 microns or less.

Specifically, Japanese Patent No. 54150395 describes the production ofslurry by grinding dried magnesium hydroxide to a specified particlesize and then mixing with water under agitation. U.S. Pat. Nos.5,762,901 and 5,514,357 describe the stabilization of slurries in whichthe slurry contains chloride ions in 0.30-0.42%, by weight on an MgObasis. These describe the use of a cationic polymer and, if required, athickening agent to stabilize the slurry, so that the slurry formed byphysical deflocculation is stable, so that it can be transported andstored without substantial agglomeration of the magnesium hydroxidesolids. U.S. Pat. No. 5,877,247 describes the stabilization of slurriesformed from solid magnesium hydroxide using a combination of one or morepolymeric dispersants and one or more water-soluble alkali metal salts.Patent EP 1009717 A4 discloses the production of stabilized magnesiumhydroxide slurry using wet milling of magnesium hydroxide granules togive control of the median particle size, controlled particle sizerange, and controlled surface area of the Mg(OH)₂ solids in the slurry.

The addition of viscosity-modifying agents and dispersants to slurriesto control viscosity, stability and dispersability, is well-establishedart. Such viscosity-modifying agents or dispersants can includedecomposable phosphates (FR 2399485); carboxylic acid type polymericsurfactants (JP 5-208810); polyanion and anion of strong acids such asHCl, H₂SO₃ or H₂SO₄ (DE 4302539); polymeric anion dispersant andwater-soluble alkali metal salt (AU 48785/93); sulphomethylatedacrylamide homopolymers or copolymers (U.S. Pat. No. 4,743,396);alkaline salts of a sulfosuccinic ester product (DE 3323730); alkalimetal silicate and hydroxide and/or mineral acid salts (J 62007439);organic or inorganic dispersants (J 61270214); xanthan gum and ligninsulphonates (CA 110 (10):7837e); carboxymethylcellulose (CA 104(6):39729k); cationic polymers (U.S. Pat. No. 4,430,248); ferroushydroxide or aluminum hydroxide (CA 79(8):44013S) and polyacrylates(U.S. Pat. No. 4,230,610).

b) Dead-Burned Magnesia (DBM).

DBM, generally in granules of about 25 mm or less, is generally producedby the calcination of the mineral magnesite. DBM is sintered, with avery low specific surface area, often below 0.1 m²/gm. When mixed withwater, the hydration of DBM to produce magnesium hydroxide is very slow,over many days and weeks. The prior art for slurries formed from DBM isfocused on activating the hydration process, dealing with the propensityof insoluble magnesium hydroxide to coat the small surface areapresented, and to slow down the reaction. The means of activationinclude wet milling to regenerate the surface, and preferably in hotwater to take advantage of the fact that the hydration reaction isthermally activated, and the use of chemical additives that areassociated with lifting the coating from the surface. Generally,viscosity modifiers and stabilizers are used to produce a thin, stablehigh-solids slurry, in the same manner required for (a).

Specifically, U.S. Pat. No. 5,487,879 A describes the process ofproduction of a stabilized, pressure-hydrated magnesium hydroxide slurryfrom ground DBM. A mixture comprising ground DBM and water is pressurehydrated to provide a pressure-hydrated slurry. The pressure-hydratedslurry is then de-agglomerated. If desired, chloride ions and cationicpolymer can be added to further stabilize the slurry. The pressure ispreferably 2-7 bar and the temperature is preferably that of wet steamat that pressure. The process is catalyzed by the introduction ofchloride ions. This patent teaches the use of magnesium chloride tocatalyze the hydration of the DBM.

As an alternative to the pressure process, the wet milling of the DBMgranules is described in the prior art. U.S. Pat. No. 5,906,804 A andEuropean Patent No. 0772570 B1 describe a process for producing a stablemagnesium hydroxide slurry in which wet grinding calcined magnesiagranules having a particle size of about 25 mm or less, and hydratingthe finely divided magnesia in a hydration zone, wherein the finelydivided magnesia is mixed with water under agitation and heat so as toproduce a magnesium hydroxide slurry having at least 80% hydration; andpassing the slurry through a second particle reduction zone so as toproduce slurry particles, wherein 90% of the slurry particles have asize less than 50 microns. A viscosity-modifying agent is added toensure a maximum viscosity of 1000 cP. The final product is described asstable, pumpable, magnesium hydroxide slurries having a solids contentof at least 40%. The viscosity-modifying agent is selected from thegroup consisting of either inorganic acids having a molecular weightless than 130 amu, or inorganic salts thereof having an alkali metal asa sole cation; or carboxylic acids having a molecular weight of lessthan 200 amu, optionally containing one or more hydroxyl groups andsalts thereof, excluding salts having alkali metal as a sole cation; orpolyhydric alcohols and carbohydrates containing two or more hydroxylgroups and having a molecular weight of less than 500 amu; or alkalineearth oxides, hydroxides and a combination thereof. This patent teachesrecirculating the parent (unhydrated solids) through a loop until theparticle is substantially consumed.

Generally, viscosity modifiers and stabilizers are used to produce athin, stable, high-solids slurry from DBM, in the same manner requiredfor (a).

c) Granular Caustic Calcined Magnesia (GCCM).

GCCM is also produced from the calcination of the magnesite. GCCMgranules may be extracted from a kiln at an earlier stage of processthan DBM. The surface area of GCCM is typically in the range of 25-60m²/gm. GCCM is also sintered, but to a lesser degree than DBM.

In the case of GCCM, the rate-limiting process for slurry formation isthe wet milling of the granules. The faster hydration is associated withthe use of shorter time milling processes compared to DBM. In commonwith slurries made from PMH and DBM, high-solids slurries requireviscosity modifiers and stabilizers to produce a thin, stable,high-solids slurry.

Specifically, the production of slurries from GCCM is described inJapanese Patent No. 5-279017 and Japanese Patent No. 5-279018. GCCM isintroduced into a hydration tank equipped with a stirrer or agitator andis simultaneously milled by steel balls or other form of abradingapparatus. Bron et al., Chemical Abstracts (CA) 68(2): 5884e (1966),refers to the hydration of magnesite-derived MgO. In this case,magnesium hydroxide was produced during boiling or short wet grinding ofthe MgO with water in a ball mill. European Patent No. 0599085 describesa process in which GCCM is comminuted in the wet state with a wetpulverizer and hydrated in the presence of an alkaline aqueous mediumthat included sodium hydroxide at an elevated temperature of not lessthan 70° C. The resultant pulverized material is classified into fineand coarse particles using a classifying means that is generally set torestrict the passage of particles in excess of 20 microns. Subsequently,the coarse particles are recycled to the wet pulverizer. By subjectingGCCM to concurrent wet pulverization and hydration in the presence of aheated alkaline aqueous medium, magnesia can be simultaneouslycomminuted and hydrated under rapid heating to produce an activemagnesium hydroxide showing a low viscosity, even at a highconcentration.

South Korean Patent No. 9301256 describes formation of active magnesiumhydrate made from light-burned magnesite that is subjected to wetcrushing with water, an alkali stabilizer inclusive of sodium hydroxide,and dispersing agent inclusive of polycarboxylate using reaction heatand crushing heat.

Generally, viscosity modifiers and stabilizers are used to produce athin, stable, high-solids slurry from GCCM, in the same manner requiredfor (a) and (b).

Powdered Caustic Calcined Magnesia (PCCM).

PCCM may be produced by simply grinding GCCM, or may be directlyproduced by the flash calcination of ground magnesite powders, or bydrying slurries produced by any of the aforementioned processes andflash calcining the dried hydroxide. Most flash calciners, however,generally have the undesirable property that some particles are exposedto very high temperatures from the hot combustion gas, and calcine andsinter quickly, so that the product has variable specific surface area,and variable hydration properties. The average properties of flashcalcined PCCM are otherwise similar to those of PCCM from grindinggranules. Indirect heating, counterflow calciners, as described bySceats and Horley, for example, in WO 2007/112496 (incorporated hereinby reference), produce uniformly calcined PCCM with minimal sinteringand a high specific surface area, which can be in the range of 100-250m²/gm with the degree of calcination of 90-98%. Such calcined PCCM hasmarkedly different surface chemical properties than PCCM produced byconventional methods.

The production of slurries from PCCM has been previously described.JP-2-48414 refers to a process of producing slurry from PCCM having asolids content of 5-70% wt % at above 50° C. under agitation, whereinsome slurry is periodically removed and replaced by hot water andmagnesia to obtain a uniform slurry density. JP 3-252311 refers to aprocess for preparing PCCM grinding the GCCM to a mean particle size of5-10 microns and then subjecting the ultra-fine powder in an acidicreaction. JP-01-212214 refers to a method of manufacturing a PCCM slurryhaving 10-50% wt % Mg(OH)₂, wherein magnesia having a mean particlediameter of less than 100 microns is hydrated in the presence of alkalimetal ions and/or alkaline earth metal ions and also in the presence ofthe hydroxide ion, nitrate ion, carbonate ion, chloride ion and/orsulphate ion. DD 272288 describes hydration of MgO resulting from MgCl₂thermal decomposition carried out by (a) pre-hydrating MgO in one ormore series or parallel connected hydration reactors; and (b) grindingin one or more series or parallel connected hydration reactors. JP03-60774 refers to the production of magnesium hydroxide slurries thatincludes the step of slaking finely pulverized light burnt magnesia thatis obtained by firing naturally produced magnesite with water withheating to 85-100° C. Sodium hydroxide is added as a hydrationaccelerator. It is known from JP 5-208810 and JP 3-252311, for example,that magnesia may be produced by calcination of magnesite followed byparticle reduction to obtain ultra-fine particles having a mean particlesize of 5-10 microns that are then hydrated to form magnesium hydroxideslurry. The hydration process can be carried out in a particle reductionzone.

It is also known to use additives to accelerate the hydration of MgO toMg(OH)₂ and/or to modify the crystal shape of the magnesium hydroxideproduct during hydration. Such additives include citric acid ormagnesium chloride (see CA 110(24):215623f), short chain carboxylicacids or corresponding salts such as magnesium acetate (JP 3-197315, JP01-131022 and DD 280745), ammonium chloride (DD 241247); magnesiumchloride, magnesium acetate, magnesium sulphate or magnesium nitrate (DD246971); inorganic or organic acids such as HCl or acetic acid or theirmagnesium salts such as magnesium chloride, or magnesium acetate(CA111(18):159019n), proprionic acid (JP 63-277510), n-butyric acid (JP63-277511), and sodium hydroxide (JP 03-60774).

JP-3-197315 refers to the production of a magnesium hydroxide slurryhaving 3-70 wt % and more preferably 20-50 wt % solids as anintermediate in the production of magnesium hydroxide crystals havinghexagonal plate-like crystals, which are obtained as a final product ofthe hydration of magnesia. These crystals are utilized as a fireretardant. JP-1-131022, which is discussed in the prior art preamble ofJP-3-197315, states that the purpose of addition of magnesium salts,such as magnesium acetate, or organic acids, such as acetic acid, is forcontrolling the rate of hydration or for controlling the growth ofmagnesium hydroxide crystals. The crystals that are obtained by thehydration process of this reference are regular in shape, therebyavoiding the formation of agglomerates.

Generally, viscosity modifiers and stabilizers are used to produce athin, stable, high-solids slurry from PCCM, in the same manner requiredfor (a), (b) and (c).

There is a need to produce low-emissions intensity slurry products tomitigate the impact of global warming. The production of PMH from brinesis energy intensive and uses hydrated lime that is generally produced inlime kilns that have significant CO₂ emissions from both the energyconsumed and from the calcination of limestone. The production of DBMand GCCM use energy-intensive kilns, that also have high CO₂ emissionsfrom both the energy consumed as well as from the calcination ofmagnesite, as does the production of PCCM from traditional flashcalciners.

In contrast, the production of PCCM using calciners, of the typedescribed by Sceats and Horley, with indirect heating of magnesiteentrained in steam produces, after steam condensation, a pure CO₂ streamthat can be liquefied and sequestered, thereby significantly reducingthe carbon footprint. Indirect heating, with counterflow, is energyefficient and, for the purpose of this specification, produces a PCCMwith a very high surface area, in the range of 100-200 m²/gm. Inaddition, this type of calciner also produces high surface area limeCaO, dolime CaO.MgO and semidolime MgO.CaCO₃. More generally, allcarbonate minerals are a mixture of limestone, dolomite and magnesite,and the calcined material from this reactor is a powder mixture of theoxides and unreacted carbonates. These powdered caustic calcinedcarbonate powders are the feedstock to produce the hydroxide slurriesdescribed in this disclosure. The high surface area calcined carbonatepowders are very reactive, and there is a need for a manufacturingprocess that can use such feedstocks for the production of slurries. Inthe development of this process for such high surface area materials,the process described herein can also be applied to the formation ofslurries from caustic calcined carbonate powders, such as PCCM, producedusing traditional flash calciners or by grinding granular causticcalcined carbonate materials, such as GCCM. The prior art describedabove for the production of slurries from PCCM cannot be used for thevery reactive powders. There is a need for a slurry production processfor powdered caustic calcined carbonates that can be applied generallyto powdered caustic calcined feedstock produced by any means, and of anycomposition. The description below is based on PCCM because magnesia isthe most difficult calcined carbonate material to slurry. The disclosureis equally applied to any powdered calcined earth carbonate, includingmixtures.

Alternatively, high surface area PCCM can be obtained by drying amagnesium hydroxide slurry formed by any of the aforementionedprocesses, and flash calcining this material at lower temperatures,preferably below 600° C. to rapidly dehydroxylate the hydroxides toreform PCCM. The lower temperature of dehydroxylation compared todecarbonation means that the sintering of the PCCM produced by thecalcination of hydroxide particles is significantly reduced, so that thesurface area is further increased. The calciners described by Sceats andHorley produce a very high surface area PCCM, of about 250 m²/gm, from adried hydroxide feed, from an initial PCCM having a surface area of lessthan 200 m²/gm. In this approach, the surface area of the PCCM ishighest when the slurry has been produced by hydrating a high surfacearea PCCM. As described above, the surface particles in slurriesproduced from higher surface area PCCM are characterized by higherconcentrations of reactive species such as peroxide and superoxide and,in many applications, these species are beneficial. Repeating the stepsof dehydration, flash calcination and hydration allows an incrementalincrease in the surface area of the PCCM produced in each step, and thusthe concentration of reactive species. The hydration process in each ofthese hydration steps requires the use of the slurry production processdescribed in this disclosure because the rate of heat release becomestoo fast for conventional slurry production processes.

Any discussion of the prior art throughout the specification should inno way be considered as an admission that such prior art is widely knownor forms part of common general knowledge in the field.

BRIEF SUMMARY Problems to be Solved

This disclosure provides a process, system, device and apparatus forproduction of hydroxide slurries from caustic calcined carbonate orhydroxide powders.

It is an object of this disclosure to overcome or ameliorate at leastone of the disadvantages of the prior art, or to provide a usefulalternative.

Means for Solving the Problem

A first aspect of this disclosure may relate to a process for producinga hydroxide slurry from caustic calcined carbonate powder, comprisingthe following steps: mixing caustic calcined carbonate powder with waterin a reactor vessel and forming a reaction mixture; applying a shearingforce to the reaction mixture using a mixing apparatus; allowing heat ofhydration to raise the temperature of the reaction mixture to near theboiling point, preferably about 95° C., and allowing steam to evaporatefrom the reaction mixture as hydration proceeds, to remove excess heatand control reaction temperature to just below or at the boiling point.

A second aspect of this disclosure relates to a process for producing ahydroxide slurry from caustic calcined hydroxide powder, comprising thefollowing steps: mixing caustic calcined hydroxide powder with water ina reactor vessel and forming a reaction mixture; applying a shearingforce to the reaction mixture using a mixing apparatus; allowing heat ofhydration to raise the temperature of the reaction mixture to near theboiling point, preferably about 95° C.; and allowing steam to evaporatefrom the reaction mixture as hydration proceeds, to remove excess heatand control reaction temperature to just below or at boiling point.

Preferably, the process is adapted for the production of a high-solidsfraction hydroxide slurry from the reaction mixture, wherein the slurryhas a relatively low resistance to shear thinning.

The process is further adapted for the production of the high-solidsfraction hydroxide slurry from the reaction mixture, wherein the slurryhas both a relatively low resistance to shear thinning and a relativelyhigh concentration of reactive chemical species such as peroxide andsuperoxide ions.

The preferred process may additionally comprise the following steps:metering an input of a viscosity modifier to enable the mixing apparatusto maintain uniform mixing under thin slurry conditions promoted by theviscosity modifier; allowing the reaction to proceed spontaneously,during boiling, until the water has ceased to boil and the temperaturedropped to a first set point; and quenching the slurry to drop thetemperature to a second set point.

The preferred powder may include ground particles, wherein the groundparticles are less than 100 microns in diameter. More preferably, theground particles have a particle size distribution in the range of 0.1up to 150 microns, and preferably with a d₉₀ of less than 100 microns.

The preferred powder may have a surface area preferably in excess of 100m²/gm, and more preferably in excess of 200 m²/gm.

The slurry may have a final solids content, after accounting for thewater loss from boiling, in the range of 40-70%. More preferably, theslurry has a final solids content, after accounting for the water lossfrom boiling, in the range of 55-65%.

Preferably, during the process, additional water is added, if required,to ensure that the final solids fraction meets the solids fractionspecification of the slurry product.

The preferred temperature of the water during the first step is within arange of 10-25° C. The preferred mixing apparatus may comprise at leastone high-shear mixer, and preferably such shear pump being external tothe reactor vessel through which the slurry is circulated by the pumpaction, and optionally further, a paddle or other similar mixer is usedto agitate the slurry in the reactor vessel.

The process may be configured to be run by a device that continuouslyoperates all of the steps of the process in a predefined order. Aviscosity modifier including, but not limited to, acetic acid ormagnesium acetate, may be added to maintain the apparent viscosity inthe range of 60-300 cP during the process, where the apparent viscosityis that of the slurry at a shear rate of preferably 200 cycles persecond.

The first set point is after the time at which the temperature hasreached a maximum, and this maximum is preferably above 90° C.Preferably, the first set point is in the range of about 85-93° C., andpreferably about 93° C., with the provision that the heat losses in thereactor apparatus are sufficiently low that the length of time to reachthe set point is less than 60 minutes. The preferred process may alsoinclude a step of quenching of the slurry at the end of processing inthe range of about 10-60° C. and preferably about 40° C. This quenchingmay be conducted by transport of the slurry to a second vessel that hasa temperature and heat capacity such that a desired quenchingtemperature is achieved.

The preferred process is adapted to yield a high-solids slurry that,after 1 month standing without agitation, exhibits syneresis ofpreferably less than 5% of the height of the storage vessel andpreferably 3%, and a toe of preferably less than 1% of the height of thestorage vessel, and that can be remixed and made to flow and pour bymild agitation.

The preferred carbonate material is calcined limestone, magnesite ordolomite.

A third aspect of this disclosure relates to a reaction apparatus forproducing a hydroxide slurry from a reaction mixture of at least causticcalcined carbonate powder or caustic calcined hydroxide powder andwater, wherein the reaction apparatus comprises: a reaction vesselhaving a first inlet adapted for receiving caustic calcined carbonatepowder and a second inlet adapted for receiving water and a controllerthat is adapted to electronically control the process within thereaction vessel; shearing apparatus positioned within the reactionvessel for shearing the reaction mixture and wherein the rate ofshearing is controlled by the controller; a viscosity sensor positionedwithin the reaction vessel adapted to supply viscosity information aboutthe reaction mixture to the controller; a temperature sensor positionedwithin the reaction vessel adapted to supply temperature informationabout the reaction mixture to the controller; and a steam outlet forrelease of steam from the reaction vessel, such that in use, thereaction is controlled by the controller so that the heat of hydrationmay raise the temperature of the reaction mixture, allowing water toboil off from the reaction mixture as hydration proceeds, and removingsteam via the steam outlet to remove excess heat and control reactiontemperature at the boiling point.

A fourth aspect of this disclosure relates to a process and apparatusfor production of hydroxide slurries from caustic calcined carbonatepowders, or caustic calcined hydroxide powders, whether such powder isderived, for example, from traditional flash calciners, from thecalciners described by Sceats and Horley, or by grinding of granularcalcined carbonate or hydroxide powders.

In one form, the disclosure provides a process of producing hydroxideslurry from caustic calcined carbonate or hydroxide powder, including:

-   -   a) mixing caustic calcined carbonate or hydroxide powder with        water in a reactor vessel;    -   b) shearing the reaction mixture; and    -   c) allowing heat of hydration to raise the temperature of the        reaction mixture to a maximum near the boiling point, and        allowing water to boil off from the reaction mixture as        hydration proceeds to remove excess heat. The maximum and        reaction temperature is bounded by the boiling point of water in        the mixture.

Optional, and preferred, process steps include one or more of:

-   -   d) metering an input of a viscosity modifier to enable the        mixing system to maintain uniform mixing under thin slurry        conditions promoted by the viscosity modifier;    -   e) allowing the reaction to proceed spontaneously, during        boiling, until the water has ceased to boil and the temperature        dropped to a first set point; and    -   f) quenching the slurry to drop the temperature to a second set        point.

Preferably, the reactor vessel is insulated to facilitate an acceleratedincrease in temperature of the reaction mixture up to the boiling point.The vessel also preferably has a steam outlet to allow escape of readysteam from the reaction vessel, such that the reaction takes place atsubstantially ambient pressure.

Further preferred aspects of control of the reaction include one or moreof:

-   -   (i) minimizing the heat losses such that the hydration heat        liberated spontaneously heats the slurry and accelerates the        hydration process such that the water boils to provide the        constant conditions at the boiling temperature and pressure to        allow the remainder of the hydration to be controlled in a        simple self-regulating manner; and    -   (ii) mixing of the water and particles to reduce the formation        of bubbles, to break up the formation of aggregates during the        slurry production, and to provide mixing so that the hydration        reaction at the surfaces of all particles can occur at a fast,        uniform rate; and    -   (iii) adding a viscosity modifier to maintain a thin slurry        during production, with the modifier being added at a rate and        to a degree necessary to allow the mixing to take place without        a substantial change in the energy consumption of the mixing        system.

The slurry is preferably quenched to terminate the hydration of residualmagnesium oxide, and cooled to ambient conditions. The process mayrequire no stabilizers to achieve the criteria for stable, readilythinned, high-solids magnesium slurries.

A further aspect of this disclosure provides for a reaction apparatusfor producing hydroxide slurry from caustic calcined carbonate powder orcaustic calcined hydroxide powder, including: a reaction vessel havingan inlet for the powder and a water inlet; shearing apparatus forshearing the reaction mixture; and a steam outlet for release of steamfrom the reaction vessel, such that in use, the reaction is controlledby allowing heat of hydration to raise the temperature of the reactionmixture, allowing water to boil off from the reaction mixture ashydration proceeds, and removing steam via the steam outlet to removeexcess heat and control the reaction temperature at boiling point.

In a fifth aspect of this disclosure, a hydroxide slurry is providedthat comprises: particles of caustic calcined carbonate or hydroxidepowder, and water; wherein the particles within the slurry have particlesize distribution in the range of 0.1 to 100 microns; and an apparentviscosity in the range of 60-200 cP.

In a sixth aspect of the disclosure, a hydroxide slurry is provided thatcomprises: particles of caustic calcined carbonate or hydroxide powderhaving a surface area preferably in excess of 100 m²/gm, or morepreferably in excess of 200 m²/gm, and water; wherein the particleswithin the slurry have particle size distribution in the range of 0.1 to100 microns; and an apparent viscosity in the range of 60-300 cP.

Preferably, the slurry is made by the processes described in theaforementioned aspects of this disclosure.

Further forms of the disclosure will be apparent from the descriptionand drawing, and from the abstract and claims.

In the context of this disclosure, the words “comprise,” “comprising,”and the like, are to be construed in their inclusive, as opposed totheir exclusive, sense, that is, in the sense of “including, but notlimited to.”

The disclosure is to be interpreted with reference to at least one ofthe technical problems described or affiliated with the background art.This disclosure aims to solve or ameliorate at least one of thetechnical problems and this may result in one or more advantageouseffects as defined by this specification and described in detail withreference to the preferred embodiments of this disclosure.

BRIEF DESCRIPTION OF THE DRAWING

Embodiments of the disclosure will be better understood and readilyapparent to one of ordinary skill in the art from the following writtendescription, by way of example only, and in conjunction with thedrawing, in which:

FIG. 1 depicts a schematic drawing of a process for production ofstable, thin, high-solids magnesium oxide slurry from powders of causticcalcined magnesia.

DETAILED DESCRIPTION

Preferred embodiments of the disclosure will now be described withreference to the accompanying drawing and non-limiting examples.

The production of a stable, thin, high-solids magnesium hydroxide slurryfrom caustic magnesia starts from the production of the PCCM. In oneembodiment, the PCCM is produced by grinding granules from aconventional kiln to achieve the desired particle size distribution. Inanother embodiment, it is produced by flash calcining pre-groundmagnesite powders in a flash calciner. These embodiments produce powderswith moderate specific surface area in the range of 20-60 m²/gm. In thepreferred embodiment, the PCCM is prepared from flash calciningpre-ground magnesite powders in an indirectly heated, counterflowreactor to produce a high surface area material, with a specific surfacearea in the range of 100-200 m²/gm. Alternatively, the PCCM is preparedby drying a slurry, and flash calcining the hydroxide powder in anindirectly heated, counterflow reactor to produce a very high surfacearea material, with a specific surface area in excess of 200 m²/gm.

In the first preferred embodiment of this disclosure, the powder ismixed into a container with water in a ratio to give the ultimatesolids/water ratio in the product, when account is taken of theconsumption of water to form the hydroxide, and the water loss fromboiling as described below. The solids and water are agitated duringmixing to prevent clumping. The temperature of the water and powder maybe at ambient, or either may have been preheated.

The basis for the process is that the energy released from hydration ofthe MgO to Mg(OH)₂ by liquid H₂O is used to heat the products of thereaction and the excess water to 100° C., and the excess heatspontaneously boils a portion of the excess water. In the ideal case ofa reactor at ambient pressure, with no heat loss and inputs at 25° C., a60% slurry can be made in which the heat released spontaneously raisesthe temperature to 100° C., and the remaining heat from the subsequentreaction spontaneously boils the water. Thus, one metric ton of 60%slurry (containing 600 kg of Mg(OH)₂ and 400 kg of water) is produced at100° C. by boiling off an additional 76 kg of water. This slurry is madeby mixing 415 kg of MgO and 661 kg of water at 25° C. From the knownthermodynamics of the reactions, the hydration of the MgO by liquidwater releases 387 MJ of heat, of which 186 MJ is used to heat thematerials to 100° C., and 201 MJ is used to heat and evaporate thewater. In the design of the reactor, with inputs at 25° C., it followsthat about 48% of the hydration reaction is complete before the boilingof the excess water occurs. Real reactors have heat losses, and mineralshave impurities, so these quantities provided above are for guidanceonly. In the prior art, the released heat is removed using heatexchangers, or for very slow reactors, the heat is lost by convection orconduction. In this disclosure, the evaporation of the water is used toremove the heat, and the boiling point of water provides a stableoperating condition for rapid processing.

The kinetics of hydration plays a very important role in the formationof slurries from DBM and PCCM. It is well established that the initialreaction rate (a) scales proportionally to the surface area of the solidparticles, and (b) has an activation energy of about 60 kJ/mol. Thismeans that the hydration reaction rate at 50° C., 75° C., and 100° C.is, respectively, 4.3, 18.1 and 57.6 times faster than that at 25° C.However, it is often observed that the rate of reaction slows downsignificantly before the reaction is complete, and this is attributed tothe low solubility of Mg(OH)₂, such that Mg(OH)₂ crystallites coats thepore surfaces. This is particularly evident from dead-burned magnesia.The solubility of Mg(OH)₂ also increases with temperature, so thiseffect becomes less important at higher temperatures. For dead-burnedmaterials, the very low porosity is such that it is believed that theMg(OH)₂ crystals formed during the reaction are separate from the parentparticle. Wet milling of DBM will remove any coating and expose newsurfaces on the particles. While the grinding process of DBM isessential, the prior art also describes the use of hot water to increasethe hydration rate. The milling conditions then determine the time toproduce the slurry. In contrast, for very high surface area PCCMparticles, the specific surface area may exceed 100 m²/gm, and there islittle evidence of pore-blocking effects. Without being limited totheory, migration of water to such CCM particles is probably not arate-limiting step because of the high porosity of the particles. Themost important observation is that the hydration reaction of CCM, in awell-stirred thermally insulated reactor, exhibits thermal runaway. Forexample, using a material with a surface area of 190 m²/gm, thetemperature of the well-stirred reactor initially rises spontaneously to50° C. over 30 minutes, and this is followed by a fast process in whichthe temperature spontaneously rises to 100° C. within 10 minutes. Theheat released by the initial hydration increases the water temperature,which increases the reaction rate so that heat released furtherincreases the temperature. This is thermal runaway. Importantly, theboiling point of water is reached preferably within thirty minutes, andthe temperature stabilizes, such that the remaining reaction can becompleted, say, with an additional 120 minutes of processing at a fixedtemperature through the release of steam. In this disclosure, theboiling of the water circumvents the need to control the temperature ofthe reactor to avert damage or hazards. Furthermore, the signature thatthe reaction is substantially complete is that boiling ceases and thetemperature begins to fall, at a rate determined by reactor heat lossesand residual hydration. It would be appreciated by a person skilled inthe art that PCCM produced with a high surface area, in the range of100-200 m²/gm will be preferred as a source of PCCM, compared to PCCMwith a surface area of 20-60 m²/gm because the processing time will beshorter and less susceptible to heat losses that might otherwise resultin the slurry not reaching the boiling point of water.

The slurries produced by a fast reaction at high temperature arecharacterized by particles that are bonded aggregates of smallcrystallites of magnesium hydroxide. These crystallites support a rangeof defect centers at the boundaries, which is believed to contribute tothe reactivity. The higher the initial surface area of the PCCM, thehigher the concentration of these defect centers.

For reasons considered below, to terminate the production, it ispreferable that the reaction is rapidly quenched to below about 60° C.when the desirable degree of reaction has been reached, i.e., asdetermined by monitoring the drop of temperature described above. It hasbeen demonstrated that the properties of such a quenched material doesnot change significantly over months. For most applications, theperformance of the slurry is not impacted by a small amount of residualoxide material, so there is no absolute requirement to achieve completehydration. When the set point is achieved, the slurry can be quenched.In a preferred embodiment, this is simply achieved by transferring theslurry batch to a steel vessel with adequate heat capacity and/orcooling, to quench the product to below about 60° C.

In summary, the evaporation of water, releasing up to about 7% of theinitial water, provides a simple means whereby the slurry can continueto be hydrated to the set point for completion without the need forexternal control or heat transfer systems during the reaction.

In the description above, a condition for the reactor is that the slurrymust be well stirred to achieve uniform kinetics.

There are several other requirements that require more detailedconsideration of the mixing process. Thus the mixing:

-   -   A) rapidly mixes the water and the particles so that the        hydration reaction occurs quickly;    -   B) rapidly mixes the water and particles so that concentration        gradients do not develop, which would otherwise slow down the        reaction and reduce the productivity of the plant. From a        quality control perspective, the removal of concentration        gradients gives a uniform product because all particles have the        same temperature and see the same aqueous environment;    -   C) breaks down aggregates of particles that otherwise form lumps        that lead to the collapse of the slurry. There is a strong        tendency of particles to agglomerate at high-solids fractions,        and the mixing is required to shear aggregates of particles. It        is noted that aggregation leads to concentration gradients,        which are to be avoided;    -   D) prevents the development of bubbles of steam in the mixture,        which otherwise leads to foaming, which also leads to an        inhomogeneous solids-liquid environment and concentration        gradients; and    -   E) comminutes the particles, so that a broader particle        distribution is developed. Comminution occurs when the particles        are subject to high-shear forces. It is noted that the hydration        process weakens the structure of the particles as the new        molecular configurations are developed. During this process, the        initial particles can fragment if subject to strong shear        forces.

Notwithstanding the concepts described above, experiments show that theformation of a stable slurry is facilitated by the use of a high-shearmixing apparatus that is capable of inducing each of the mechanismsdescribed above. In more general terms, the formation of a stable slurryis rendered more difficult to achieve without the use of such ahigh-shear mixing apparatus. In the preferred embodiment, the shearmixing pump is external to the reactor and draws the slurry from thebase of the reactor and returns the sheared slurry to the top of thereactor. A smaller pump is used to agitate the slurry in the reactor. Itis observed that the reaction rate can be moderated, if required, by thesettings of the high-shear mixing apparatus. It is stressed that anobjective of this disclosure is to minimize the use of dispersion agentsand the like, because the prior art describes instances in which theseagents interfere with the applications of the slurries.

The comminution of the particles during the slurry production processhas been observed during the process by sampling and measuring thechange of the particle size distribution during the course of thereaction. It is believed that the stability of high-solids slurry isenhanced if the particle size distribution is broad. This broadening hasbeen observed during the slurry formation using the high-shear mixingapparatus, and is likely to positively contribute to the stability ofthe slurry. Preferably, the particle size distribution of the raw feedshould be a broad distribution.

In summary, the mixing of the solids is preferably accomplished using ahigh-shear mixing apparatus that substantially dissipates concentrationgradients, agglomerates, steam bubbles and induces comminution.

All high-solids magnesium hydroxide slurries exhibit non-Newtonianviscoelastic properties, as shown by the formation of a gel, to somedegree. The requirement of the gelled slurry is that it exhibits littleresistance to thinning, and to that extent, it can be classified as athin slurry. During production, the slurry must be agitated sufficientlyto break down the gel structure so that the slurry can feed to thehigh-shear mixing apparatus described above. Post production, the slurrymust exhibit a low resistance to shear thinning so that gentle agitationthins the slurry, to enable the slurry to be pumped or poured forapplication. The means of thinning of magnesium hydroxide slurries iswell described in the prior art, and for high-solids fraction slurries,the approach of using a viscosity modifier or dispersion agent is commonto all the processes previously described. That is, the use of aviscosity modifier is a factor to be considered independently ofmaterials and method used to form the slurry. The preferable viscositymodifier is one that is low cost, and added in small amounts, typically<1%. The prior art shows that soluble salts are commonly used for thisrole. It is noted that the solubility of magnesium hydroxide is low, andat the pH of 10.4, the ionic strength of the water is not very high. Apreferred approach to increase the ionic strength is to use an acid,such as acetic acid, which reacts essentially completely with themagnesium hydroxide to form magnesium acetate ions, which act as theviscosity modifier. The pH of such a slurry is lowered to about 9.5 as aresult of the ionic strength, and this pH increases back to 10.4 whenthe slurry is diluted.

The stability of the slurry is, as described in the prior art, animportant characteristic. Measurements during the production of theslurry show that the stability of the slurry increases during thehydration process. That is, samples of slurry extracted from the reactorduring the early stages of hydration immediately collapse, while samplestaken at later stages take progressively longer to settle and, towardthe end of the reaction, the slurry does not settle on the timescale ofmonths. These characteristics do not apparently change during cooling ofthe sample. The evolution of the slurry stability is a complex processthat is linked to the degree of hydration, the mixing process,especially shear, and the use of viscosity modifiers. Importantly, thereis no adverse effect of boiling water on the slurry characteristicsprovided that the water content is managed to account for the loss.

The embodiment of the process shown in FIG. 1 shows a batch reactor forthe production of a magnesium hydroxide slurry from PCCM. The batchprocess starts with filling the reactor vessel 100 with preferably coldwater 101, and then the PCCM 102 is metered into the water, preferablyover a 10-15-minute timescale. The reactor is preferably insulated. Theslurry 103 is stirred by a paddle 104 and a portion of the material issheared by a shear pump 105 after routing through valve 106 at the baseof the reactor. The sheared slurry is returned to the reactor near thetop of the slurry surface at 107. As the reaction proceeds, thetemperature of the slurry in the reactor rises due to the exothermichydration process. As the slurry begins to gel, a viscosity modifier108, such as acetic acid, is metered into the reactor to the apparentviscosity, so that the amount of modifier is just sufficient to maintainthe apparent viscosity at a sufficiently low level that the paddlestirrer 104 and the shear pump 105 can operate within theirspecifications. In addition, as the reaction proceeds, the waterapproaches the boiling point, slightly below 100° C., and steam 109 isejected from the reactor through the stack 110. The mass loss of steamis preferably measured. The viscosity, at one or more shear rates ismeasured, along with the temperature and the mass flow of slurry throughthe shear mixer. When the reaction is nearly complete, the boilingceases and the temperature in the reactor begins to fall. Samples of theslurry may be taken and important properties, such as the stability,zeta potential and the viscosity, are measured to determine that thereaction has progressed to the point that a thin, stable slurry has beenobtained. The shear pump 105 is turned off, and the slurry 111 isdrained from the reactor at the base through valve 106. The slurry ispreferably quenched during transport to a vessel (not shown), in whichthe transport pipes and/or the vessel has either the required heatcapacity, or is cooled, so that the slurry rapidly cools to below 60° C.When quenched in this manner, the thin, stable high-solids slurry isformed with the desirable attributes. The slurry can be left to cool toambient temperatures.

Process control is thereby simplified and costs reduced by using boilingpoint as a boundary to control the process temperature and, optionally,by using a simple quenching mechanism to stop the reaction.

The simplicity of the process allows the establishment of transportableslurry plants that can be conveniently located relative to the site ofproduction of the magnesium oxide powder and the sites of consumption toreduce the costs of transporting slurries over long distances. Forexample, the disclosure may be embodied in a compact apparatus that canbe stationed in processing plants that are distant from the source ofproduction of the magnesite powder.

The slurries can be produced from a wide variety of caustic calcinedcarbonate and hydroxide materials and mixtures thereof. Such slurriescan have substantially different chemical properties that depend on thesurface area of the powder.

Preferably, the slurry forming part of the first preferred embodiment ofthis disclosure may include a slurry of a predetermined viscosity.Specifically, the viscosity of the slurry may be sufficient to allow forthe slurry to be sprayed onto the walls of a sewage pipe so as to coatthe interior of the pipe. Preferably, the viscosity of the slurry issufficiently high enough to allow the slurry to adhere to the walls ofthe pipe without falling off, while maintaining a viscosity low enoughto allow the slurry to be pumped and applied to the walls by a pumpingapparatus or spraying machine.

In this specification, the word “comprising” is to be understood in its“open” sense, that is, in the sense of “including,” and thus, notlimited to, its “closed” sense, that is the sense of “consisting onlyof” A corresponding meaning is to be attributed to the correspondingwords “comprise,” “comprised,” and “comprises” where they appear.

It will be understood that the disclosure defined herein extends to allalternative combinations of two or more of the individual featuresmentioned or evident from the text. All of these different combinationsconstitute various alternative aspects of the disclosure.

While particular embodiments of this disclosure have been described, itwill be evident to those skilled in the art that this disclosure may beembodied in other specific forms without departing from the essentialcharacteristics thereof. The present embodiments and examples are,therefore, to be considered in all respects as illustrative and notrestrictive, the scope of the invention being indicated by the appendedclaims rather than the foregoing description, and all changes that comewithin the meaning and range of equivalency of the claims are,therefore, intended to be embraced therein. It will further beunderstood that any reference herein to known prior art does not, unlessthe contrary indication appears, constitute an admission that such priorart is commonly known by those skilled in the art to which thedisclosure relates.

Although the disclosure has been described with reference to specificexamples, it will be appreciated by those skilled in the art that thedisclosure may be embodied in many other forms in keeping with the broadprinciples and the spirit of the disclosure described herein.

This disclosure and the described preferred embodiments specificallyinclude at least one feature that is applicable to industry.

The claim defining the invention are as follows:
 1. A process forproducing a hydroxide slurry from caustic calcined carbonate powder,comprising the following steps: a) mixing the powder with water in areactor vessel and forming a reaction mixture; b) adding a viscositymodifier to the reaction mixture; c) applying a shearing force to thereaction mixture using a mixing apparatus; d) allowing heat of hydrationto raise a temperature of the reaction mixture to a temperature in arange of 85-95° C.; and e) allowing steam to evaporate from the reactionmixture as hydration proceeds, to remove excess heat and control amaximum reaction temperature to be in the range of 85-95° C., whereinthe hydroxide slurry including the viscosity modifier formed by steps(a)-(e) has a pH of about 9.5.
 2. The process for producing thehydroxide slurry of claim 1, wherein the powder is selected from thegroup consisting of calcined limestone, calcined magnesite, and calcineddolomite.
 3. The process for producing the hydroxide slurry of claim 1,wherein the process yields a high-solids fraction hydroxide slurry that,after 1 month standing without agitation, exhibits syneresis of lessthan 5% of a height of a storage vessel, and a toe of less than 1% ofthe height of the storage vessel, and that can be remixed and made toflow and pour by mild agitation.
 4. The process for producing thehydroxide slurry of claim 1, wherein: mixing the powder with water in areactor vessel comprises providing the caustic calcined carbonate powderthrough a first inlet of the reactor vessel and providing the waterthrough a second inlet of the reactor vessel, the reactor vesselcomprising a controller that is adapted to electronically control theprocess within the reactor vessel, a viscosity sensor positioned thereinand adapted to supply viscosity information about the reaction mixtureto the controller, and a temperature sensor positioned therein andadapted to supply temperature information about the reaction mixture tothe controller; applying a shearing force to the reaction mixturecomprises applying a shearing force to the reaction mixture with ashearing apparatus positioned external to the reactor vessel, whereinthe rate of shearing is controlled by the controller; and allowing steamto evaporate from the reaction mixture comprises evaporating steamthrough a steam outlet of the reactor vessel, such that, in use, thereaction is controlled by the controller so that the heat of hydrationmay raise the temperature of the reaction mixture, allowing water toboil off from the reaction mixture as hydration proceeds, and removingsteam via the steam outlet to remove excess heat and control reactiontemperature at boiling point.
 5. The process for producing the hydroxideslurry of claim 1, wherein mixing the powder with water in a reactorvessel comprises mixing particles of caustic calcined carbonate powderwith the water, wherein the particles have particle size distribution inthe range of 0.1 to 150 microns; and wherein the reaction mixture has anapparent viscosity in the range of 60-300 cP.
 6. The process forproducing the hydroxide slurry of claim 1, wherein adding the viscositymodifier to the reaction mixture comprises adding the viscosity modifierto the reaction mixture to maintain an apparent viscosity in the rangeof 60-300 cP during the process, where the apparent viscosity is that ofthe high-solid fraction hydroxide slurry at a shear rate of preferablyof 200 rpm.
 7. The process for producing the hydroxide slurry of claim1, wherein the process produces a high-solid fraction hydroxide slurryfrom the reaction mixture, wherein a weight of solids in the high-solidfraction hydroxide slurry is in a range of 40-70% and wherein thehigh-solid fraction hydroxide slurry has a relatively lower resistanceto shear thinning with respect to a hydroxide slurry having a weight ofsolids less than 40%.
 8. The process for producing the hydroxide slurryof claim 7, wherein the process produces the high-solids fractionhydroxide slurry from the reaction mixture, wherein the high-solidsfraction hydroxide slurry has a relatively higher concentration ofchemically reactive species with respect to the hydroxide slurry havingthe weight of solids less than 40%.
 9. The process for producing thehydroxide slurry of claim 8, wherein the powder has a surface areabetween 100-200 m²/gm or greater than 200 m²/gm.
 10. The process forproducing the hydroxide slurry of claim 7, wherein a temperature of thewater during step (a) is within a range of 10-25° C.
 11. The process forproducing the hydroxide slurry of claim 7, wherein the mixing apparatuscomprises at least one high-shear mixer.
 12. The process for producingthe hydroxide slurry of claim 7, wherein the process additionallycomprises the following steps: f) metering an input of the viscositymodifier to enable the mixing apparatus to maintain uniform mixing underthin slurry conditions promoted by the viscosity modifier; g) allowingthe reaction to proceed spontaneously, using evaporation to balance theheat release, until a temperature of the water has reached a maximumtemperature and the temperature of the reaction mixture has dropped to afirst set point; and h) quenching the high-solid fraction slurry to dropthe temperature of the reaction mixture to a second set point.
 13. Theprocess for producing the hydroxide slurry of claim 12, wherein themaximum temperature of the water is near the boiling point of thereaction mixture.
 14. The process for producing the hydroxide slurry ofclaim 13, wherein the first set point is in the range of 85-95° C. 15.The process for producing the hydroxide slurry of claim 12, wherein thepowder includes ground particles having a particle size distribution ofless than 100 microns.
 16. The process for producing the hydroxideslurry of claim 12, wherein the powder includes ground particles havinga particle size distribution in the range of 0.1 to 150 microns.
 17. Theprocess for producing the hydroxide slurry of claim 12, wherein theslurry has a final solids content, after accounting for a water lossfrom boiling, in a range of 40-70% by weight of solids.
 18. The processfor producing the hydroxide slurry of claim 17, wherein the slurry has afinal solids content, after accounting for the water loss fromevaporation, in a range of 55-60% by weight of solids.
 19. The processfor producing the hydroxide slurry of claim 12, further comprisingadding water during at least one of steps (a)-(h) to ensure that a finalsolids fraction is in a range of 40-70% by weight of solids.
 20. Theprocess for producing the hydroxide slurry of claim 12, furthercomprising continuously repeating steps (a)-(h) in a predefined order.21. The process for producing the hydroxide slurry of claim 12, whereinthe first set point is in the range of about 85-95° C., and wherein heatlosses in the reactor apparatus are sufficiently low that the length oftime to reach the first set point is less than 60 minutes.
 22. Theprocess for producing the hydroxide slurry of claim 12, whereinquenching the high-solid fraction hydroxide slurry comprisestransporting the high-solid fraction hydroxide slurry to a second vesselthat has a temperature and heat capacity such that achieved temperatureof the high-solid fraction hydroxide slurry drops to the second setpoint.
 23. The process for producing the hydroxide slurry of claim 22,wherein the second set point is in the range of 10-60° C.
 24. Theprocess for producing the hydroxide slurry of claim 22, wherein thesecond set point is 40° C.